This invention relates to sets (or arrays) of physical mass storage devices that perform as a single logical mass storage device. More particularly, this invention relates to methods and apparatus for keeping track of when a particular member of the set (or array) is a valid member. As used herein, the terms "set" and "array" are synonymous.
Mass storage devices--i.e., tape and disk drives, and more commonly disk drives--are the primary storage devices for permanent data in electronic digital computer systems. It has been known to use a set of physical mass storage devices to function as one larger logical mass storage device. This might be done for several reasons. First, if one member of the set fails, the entire set need not be disabled, and a majority of the data on the logical device remains available, while new data can still be stored. Second, the access time for data may be reduced because any one read/write head--in the case of the disk drives--or tape reel--in the case of tape drives--need not move as far to find the particular data it is seeking if the physical devices are small compared to a single physical device of the same storage capacity as the set. This last factor has become more important recently in the case of disk drives in particular as the cost, speed and reliability of small disk drives have improved faster than those of larger drives. Therefore, although the present invention is applicable to any mass storage technology--i.e., disk, tape or any other technology, the discussion below will refer only to disk drives, it being understood that the scope of the present invention is not limited to disk drives.
When a set of physical disk drives is used as a single logical drive, care must be taken to maintain the integrity of data that is spread over several drives. For example, it has been known to use checksums and other error correction techniques with sets of drives to allow the reconstruction, from the active drives in a set, of data stored on a drive that is disabled or removed for other reasons.
If a set of drives can operate with one or more drives removed, data may be written to the set while the drive is removed. Depending on how data is stored across the set, the data on the removed drive may not be valid when the drive is reinstalled until it is updated or regenerated. Therefore, it is important to know, when a drive is reinstalled, whether or not it is a valid member of the set. In one known system, the computer system maintains a table or list of physical serial numbers for the members of the set and can read the serial numbers of each member, determining from the list whether the drives are valid members. However, the table of physical serial numbers in such a system is maintained in volatile memory, and is not protected in the event of a power failure. If interrupted, the system would fail, and the membership set information would be lost. Moreover, in such a system, the table or list in the computer system must be created manually and must be updated to reflect the removal of a drive and its reconnection. While such updating of the table or list might be done automatically, it has generally been done manually, by an operator. Even when the update is automatic, it requires the involvement of the central processor. Also, if the membership information is not stored in the set of drives, the information may be lost if control over the set is transferred from the device in which the table or list has been maintained to another device (e.g., if the first device fails).
It would be desirable to be able to provide a "signature" system for a set of physical disk drives functioning as one logical drive which would automatically reflect the current membership status of any drive in the set and which would automatically be updated and remain valid if an update was interrupted by a power failure.
It would also be desirable to be able to provide such a system that did not require manual intervention by an operator and did not require the involvement of the central processor.
It would also be desirable to be able to provide such a system that stored membership information on the drive or drives.